The Elusive OTA ( Operational Transconductance Amplifier ). The mythic discontinued current amplifier of the late 60's early 70's. Used in several early synths and guitar pedals of the past and even modern modular filters and pedal kits today!
Picture by Jpoo
References:
Fascinating OTA story
Muffwiggler
OB filter clone OTA style
STE440 modular Filter
::: IF :::